1. Field of the Invention
This invention relates to the synthesis of N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester (neotame) using L-xcex1-aspartyl-L-phenylalanine 1-methyl ester (aspartame) precursors. This method of producing neotame results in high purity and is more simple and more economical than the conventional preparation of neotame.
2. Related Background Art
N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester (neotame) is a high potency dipeptide sweetener (about 8000xc3x97 sweeter than sucrose) that has the formula 
The chemical synthesis of neotame is disclosed in U.S. Pat. Nos. 5,480,668, 5,510,508, 5,728,862 and WO 00/15656, the disclosure of each of which is incorporated by reference herein.
U.S. Pat. Nos. 5,510,508 and 5,728,862 describe the synthesis of neotame by hydrogenation of a mixture of aspartame and 3,3-dimethylbutyraldehyde with a catalyst such as Pd on carbon. This synthesis is represented by the following equation. 
The aspartame used in this synthesis is produced by its precursors. For example, aspartame may be produced by neutralization of aspartame hydrochloride followed by crystallization as described in U.S. Pat. No. 5,266,719, the disclosure of which is incorporated by reference herein. Such a neotame process requires the reaction of pure isolated aspartame with the aldehyde to produce neotame. Therefore, it would be economically advantageous to use aspartame precursors directly in neotame synthesis without having to first isolate aspartame.
International Patent Publication No. WO 00/15656 attempts to eliminate some of these complicated process steps. WO 00/15656 describes the formation of neotame by hydrogenation of a mixture of 3,3-dimethylbutyraldehyde and Z-aspartame (N-benzyloxycarbonyl-L-xcex1-aspartyl-L-phenylalanine-1-methyl ester) in a methanolic solvent.
WO 00/15656 discloses that the neotame obtained by this method has a purity as low as 87% with significant amounts of known and unknown impurities.
Since neotame is mainly employed in foods for human consumption, it is extremely important that neotame exist in a highly purified state. Any impurity  greater than 0.1% must be structurally characterized and subjected to safety studies. In this regard, it is clear that the neotame produced by the method of WO 00/15656 is not acceptable with respect to purity.
Thus, it is clear that there is a need to economically and efficiently produce pure N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester.
The present invention relates to the efficient, low cost and high purity synthesis of N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester (neotame). According to one embodiment of the present invention, neotame is synthesized by reacting an acid salt of aspartame and 3,3-dimethylbutyraldehyde in a solvent or a mixture of solvents under hydrogenation conditions with a catalyst. The acid salt of aspartame may be an isolated compound or its wet cake. In certain preferred embodiments, hydrobromic, sulfuric, phosphoric, citric, acetic or hydrochloric acid salts of aspartame are employed.
In a second embodiment of the present invention, neotame is synthesized by hydrogenating a mixture of N-protected aspartame and 3,3-dimethylbutyraldehyde in a solvent or a mixture of solvents with a catalyst. The protecting groups are those that can be cleaved by hydrogenolysis. They include, without limitation, carbamate, amides, benzylidenes, benzyl and silyl.
The present invention relates to the synthesis of N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester (neotame) by hydrogenation of a mixture of 3,3-dimethylbutyraldehyde and a precursor of L-xcex1-aspartyl-L-phenylalanine 1-methyl ester (aspartame) with a catalyst. More specifically, an acid salt of aspartame (isolated or generated in situ) or protected aspartame is used to produce neotame, thereby eliminating the need to isolate aspartame prior to its combination with 3,3-dimethylbutyraldehyde.
According to the first embodiment of the present invention, N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester is synthesized by reacting an acid salt of L-xcex1-aspartyl-L-phenylalanine 1-methyl ester and 3,3-dimethylbutyraldehyde in a solvent or a mixture of solvents under hydrogenation conditions, i.e., in the presence of hydrogen, with a catalyst. Optionally, the acid salt of L-xcex1-aspartyl-L-phenylalanine 1-methyl ester is pretreated with a base; alternatively, a base is included in the reaction mixture.
Suitable acid salts of aspartame include, without limitation, hydrobromic, sulfuric, phosphoric, citric, acetic and hydrochloric acid salts of L-xcex1-aspartyl-L-phenylalanine 1-methyl ester. The acid salt of aspartame used in the present inventive process can be a wet cake or a dry cake. The acid salt of aspartame can also be used in situ from its precursors by the acidic hydrolysis of an N-protected aspartame, for example, by treatment of N-formyl aspartame, N-BOC aspartame or other N-protected aspartame with HCl or another acid. The N-protected aspartame may be, without limitation, acetyl, benzoyl, benzyloxycarbonyl, (p-chlorobenzyl)oxycarbonyl, (p-bromobenzyl)oxycarbonyl, (p-nitrobenzyl)oxycarbonyl, tert-butyloxycarbonyl, formyl, (p-methoxybenzyl)oxycarbonyl (MOZ) or p-toluene sulfonyl (Ts). One of ordinary skill in the art would readily recognize that both the meta- and ortho-forms of the above-listed para-groups are also suitable for use in the present invention.
Generally the concentration of the acid salt of aspartame in the solvent is in a range of about 2% to about 20%, more preferably about 7% to about 15%.
The acid salt of aspartame is optionally pre-treated with a base. Suitable bases include, without limitation, sodium carbonate, potassium carbonate and ammonium hydroxide. Alternatively, a base may be included in the reaction mixture. The base may be used in amount of about 0.80 to about 1.2 equivalent of the acid in the acid salt of aspartame.
According to the second embodiment of the present invention, N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester is synthesized by reacting an N-protected L-xcex1-aspartyl-L-phenylalanine 1-methyl ester intermediate, 3,3-dimethylbutyraldehyde and a catalyst in a solvent or a mixture of solvents in the presence of hydrogen to produce N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester.
The N-protected aspartame suitable for hydrogenolysis use in the present invention comprises a protecting group. Suitable protecting groups include, without limitation, carbamates such as benzyloxycarbonyl, (p-chlorobenzyl)oxycarbonyl, (p-bromobenzyl)oxycarbonyl,(p-methoxybenzyl)oxycarbonyl (MOZ), (p-nitrobenzyl)oxycarbonyl, formyl, benzyl and silyl.
The N-protected aspartame used in the present inventive process can be a wet cake or a dry cake. Protected aspartame can be prepared by any known chemical or enzymatic method.
Generally the concentration of the N-protected aspartame in the solvent is in a range of about 2% to about 20%, more preferably about 7% to about 15%.
Solvents suitable for use in either embodiment of the present invention include, without limitation, ethanol, ethyl acetate, acetonitrile, dioxane, methanol, isopropanol, isobutyl methyl ketone, tetrahydrofuran, cyclohexane, toluene, dimethylformamide (DMF), water and mixtures thereof. The solvent can be added to a dry cake of an acid salt of aspartame or N-protected aspartame. Alternatively, the solvent may be used in situ in the formation of an acid salt of aspartame or N-protected aspartame, or it may be added to a reaction mixture.
The catalyst suitable for use in either embodiment of the present invention may be selected from catalysts based on palladium or platinum including, without limitation, platinum on activated carbon, palladium on activated carbon, platinum black or palladium black. Other catalysts include, without limitation, nickel on silica, nickel on alumina, Raney nickel, ruthenium black, ruthenium on carbon, palladium hydroxide on carbon, palladium oxide, platinum oxide, rhodium black, rhodium on carbon and rhodium on alumina. The catalysts based on palladium or platinum are preferred.
The catalyst is present in an amount effective to produce neotame in an acceptable rate and yield. Generally, the weight ratio of catalyst (on a dry basis) to aspartame is about 0.01:1 to about 0.25:1, preferably about 0.10:1. It is important to note that about a 10% catalyst loading is required to minimize the undesirable yield of dialkylated aspartame.
3,3-Dimethylbutyraldehyde is a readily available starting material. According to the present invention, 3,3-dimethylbutyraldehyde can be added slowly or all at once to the reaction mixture. When the aldehyde is gradually added, typically it is added over the course of about 2 to 8 hours, preferably from about 4 to 6 hours.
3,3-Dimethylbutyraldehyde and the aspartame precursor are typically combined in a substantially equivalent molar ratio, i.e., about 1:0.95 to 1:1. Excess molar amounts of aspartame are not preferred due to waste and cost. Higher molar amounts of the aldehyde are likely to lead to the generation of impurities. Further, the 3,3-dimethylbutyraldehyde used in the present process should be highly pure. Small impurities in the 3,3-dimethylbutyraldehyde may produce odor. Higher molar ratios of aldehyde may cause the entrapment of the aldehyde during subsequent crystallization of neotame and produce odor; alternatively, excess aldehyde may be oxidized to the corresponding t-butyl acetic acid which also produces odor. The odor can be removed by washing the final product with organic solvents (such as heptane, ethyl acetate, t-butylmethyl ether, hexane, etc.) or by extruding the final product. The excess aldehyde may also react with neotame to give dialkylated imidazolidinone. This may also be ultimately crystallized along with neotame and will hydrolyze to give neotame and aldehyde.
3,3-Dimethylbutyraldehyde and the acid salt of aspartame or the N-protected aspartame are reacted for a time and at a temperature sufficient to produce neotame. Generally, the time ranges from about 1 to about 24 hours, preferably from about 2 to about 4 hours after addition of the 3,3-dimethylbutyraldehyde is complete. If the 3,3-dimethylbutyraldehyde is added to the reaction mixture all at once, then the time sufficient to produce neotame preferably ranges from about 6 to about 24 hours. Generally, the temperature sufficient to produce neotame according to the present invention ranges from about 20xc2x0 C. to about 60xc2x0 C., preferably from about 22xc2x0 C. to about 40xc2x0 C.
The reactions of the present invention are carried out in the presence of hydrogen. Generally, the pressure of the hydrogen ranges from about 5 psi to about 100 psi, preferably from about 30 psi to about 50 psi.
Either of the above-described embodiments may also include additional steps. Such additional steps include, without limitation, catalyst removal, solvent concentration adjustment, holding, seeding, cooling (crystallization), and neotame isolation.
The catalyst may be separated by a variety of solid-liquid separation techniques that include, without limitation, the use of sparkler, crossflow, nutsche, basket, belt, disc, drum, cartridge, candle, leaf and bag filters. Furthermore, catalyst separation performance may be enhanced through the use of gravity, pressure, vacuum and/or centrifugal force. Additionally, the catalyst separation rate and removal efficiency may be enhanced through the use of any number of various filter media that include, without limitation, woven cloth fabrics, woven metal fabrics, porous metal substrates and synthetic or naturally occurring membranes. The separation device and media can be permanent, replaceable or disposable. The catalyst solid alone may be separated, or separation may be assisted by the use of porous cellulosic fiber or diatomaceous silica type filter aids, which are used as a media precoat and/or directly with a catalyst slurry. The separation device can be operated in an automated or manual mode for solid media washing, solid discharging and/or solid and media back flushing. The catalyst can be washed and discharged from the filter media using gas, liquid or mechanical means. The catalyst alone or catalyst with filter aid can be partially or totally recycled for used in subsequent hydrogenation reactions.
The reaction mixture, if water is present, may be held for a time and at a temperature sufficient to hydrolyze dialkylated imidazolidinone to a-neotame and 3,3-dimethylbutyraldehyde. The reaction mixture is generally held for about 0.5-24 hours at a temperature of about 20-50xc2x0 C. In a preferred embodiment of the present invention, the reaction mixture is held for about 2-4 hours.
Typically crystallization of neotame is accomplished by cooling the mixture to about 0-25xc2x0 C., preferably to about 5-10xc2x0 C., over the course of about 0.5-2 hours, preferably about 1-2 hours.
Seeding prior to or during crystallization can initiate a controlled crystal growth rate according to the present invention. Hence, the reaction mixture may optionally be seeded in an amount from 0.0001%-10%, by weight of the N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester in the solution, preferably from 0.1% to 1% and most preferably from 0.1% to 0.5%. Seeding is typically performed at 25-35xc2x0 C. and preferably at 28-30xc2x0 C.
The reaction mixture or the solution containing neotame may be unstirred or stirred according to any embodiment of the present invention.
Crystallized neotame may be separated from the solvent solution by a variety of solid-liquid separation techniques that utilize centrifugal force, that include, without limitation, vertical and horizontal perforated basket centrifuge, solid bowl centrifuge, decanter centrifuge, peeler type centrifuge, pusher type centrifuge, Heinkel type centrifuge, disc stack centrifuge and cyclone separation. Additionally, separation may be enhanced by any of pressure, vacuum, and gravity filtration methods, that include, without limitation, the use of belt, drum, nutsche type, leaf, plate, Rosenmund type, sparkler type, and bag filters and filter press. Operation of the neotame solid-liquid separation device may be continuous, semi-continuous or in batch mode. The neotame solid may also be washed on the separation device using various liquid solvents, including, without limitation, water, methanol and mixtures thereof. The neotame solid can also be partially and totally dried on the separation device using any number of gases, including, without limitation, nitrogen and air, to evaporate residual liquid solvent. The neotame solid may be automatically or manually removed from the separation device using liquids, gases or mechanical means by either dissolving the solid or maintaining the solid form.
The neotame synthesized according to the present invention may be purified by any known method including, but not limited to, the following methods. U.S. Pat. No. 5,728,862 outlines a purification method by which neotame is precipitated out of an aqueous/organic solvent solution, wherein the aqueous/organic solvent solution has an amount of organic solvent of about 17% to about 30% by weight. Copending U.S. patent application Ser. No. 09/448,671, filed on Nov. 24, 1999, relates to methods of purifying neotame by crystallization in a variety of organic solvent/aqueous organic solvent mixtures; each of these methods involves the use of an organic solvent and water mixture and solvent distillation. Copending U.S. patent application Ser. No. 09/449,314, filed on Nov. 24, 1999, relates to methods of purifying neotame using chromatography.
The neotame synthesized according to the present invention is the monohydrate, which may be dried to produce an anhydrous form.
The crystallized and isolated neotame solid may be further purified by a variety of drying methods. Such methods are known to those skilled in the art and include, but are not limited to, the use of a rotary vacuum dryer, fluid bed dryer, rotary tunnel dryer, plate dryer, tray dryer, Nauta type dryer, spray dryer, flash dryer, micron dryer, pan dryer, high and low speed paddle dryer and microwave dryer.
The above-described process of the present invention achieves a number of advantages as compared to conventional neotame synthetic routes. In particular, complicated processing steps to isolate aspartame prior to combining it with 3,3-dimethylbutyraldehyde are eliminated. On a manufacturing scale, this results in processing time savings, as well as a significant cost savings.
The Examples which follow are intended as an illustration of certain preferred embodiments of the invention, and no limitation of the invention is implied.